Products such as wearable products may have an “instant-on” feature that differentiates these products from other devices (e.g., tablets, mobile phones etc.,), where the device does not function until the battery has reached a minimum functional voltage for the device, which can be several minutes.
In the case of wearables, device functionality may be expected when connected to a battery charger (wired or wireless). With instant-on capability, regardless of a battery condition of a battery when connected to a charger source, the device will become operational substantially immediately, as perceived by a user. Operational, as used here, can include any level of performance, e.g., from complete use of all device features to use of a subset of the device features.
Instant on operation may be accomplished via a power multiplex switch. In normal operation, supply rails for the device are powered from the battery, which is disconnected from a charging source, e.g., a power source that is available to provide power as needed, and can be used to charge the battery. If the battery is unable to supply one of the rail voltages (due to partial discharge and corresponding low battery voltage), then all of the supply rails are to be supplied from another source, e.g., the charging source, which may result in inefficiencies in power usage and in area to be consumed by the power multiplex switch. That is, unless the battery voltage exceeds all of the instant-on voltages to be supplied, power to supply each rail voltage will come from the charging source. In order to supply one or more small instant-on voltages, a step-down of voltage (e.g., from charging source to each smaller instant-on voltage) may be needed for each instant-on voltage to be supplied, which may result in significant wasted power.